A motor control apparatus for driving a brushless (direct current) DC motor are disclosed in JP-A-H11-275885 and JP-A-2004-19461. The motor control apparatus include an inverter circuit for switching a drive current provided to armature windings of the motor at a predetermined timing so that the motor can rotate. The motor control apparatus reduces noise and vibration generated from the motor.
In the motor control apparatus disclosed in JP-A-H11-275885, a rotational speed range of the motor is divided into multiple speed regions and multiple switching timings corresponding to each speed region are prestored in a memory device. For example, when the motor is driven in a first speed region, a first switching timing corresponding to the first speed region is read from the memory device and the inverter circuit switches the drive current at the first switching timing. The switching timings are set to reduce vibration of the motor, the motor assembly, or a structure around the motor as much as possible. The vibration is due to torque ripple caused by switching the drive current. In each speed region of the rotational speed range, thus, the noise and vibration due to the torque ripple are reduced.
In the motor control apparatus disclosed in JP-A-2004-19461, an average voltage corresponding to the rotational speed of the motor is multiplied by a control value corresponding to a rotational angle (load torque) of the motor. In such an approach, a drive torque generated at each rotational angle can track the load torque to reduce vibration due to a difference between the drive torque and the load torque.
It is known that when resonance frequencies of the motor, the motor assembly, or the structure around the motor are equal to orders, i.e., harmonics of the rotational speed of the motor, the order components may cause the noise and vibration.
The motor control apparatus disclosed in JP-A-H11-275885 is for reducing the noise and vibration due to the torque ripple and the motor control apparatus disclosed in JP-A-2004-19461 is for reducing the noise and vibration due to the difference between the drive torque and the load torque. In the motor control apparatus disclosed in JP-A-H11-275885 and JP-A-2004-19461, therefore, it is difficult to reduce the noise and vibration due to the orders of the rotational speed of the motor.